Correct syntaxt to call object.draw_type = 'BOUNDS' as pie command?
read wm.py I promise this will be an eye-opener.

This has nothing to do with pies, pies are just specialized menu representations.  The property you are setting is an Enum ( a simple drop down in the UI is generally a representation of an Enum )

If you want to set the value of an Enum but don't want to present a dropdown, then you use wm.context_set_enum. 

    if context.active_object:
        obj = context.active_object
        set_draw_type = layout.operator("wm.context_set_enum", text="Object Solid", icon="MESH_CUBE")
        set_draw_type.data_path = "object.draw_type"
        set_draw_type.value = 'BOUNDS'


wm.context_set_enum is defined in the class WM_OT_context_set_enum, 
which delegated the execute function to execute_context_assign (also found in wm.py)